The present invention relates to a reactor tube for forming a high-temperature fluid-wall reactor, and to the process for enabling such a reactor tube to form an integral fluid wall for preventing reactants from contacting the interior surface of the tube. The reactor of the present invention may utilize radiation coupling, so that electrodes radiate heat to the reactor tube, which is heated to incandescence and accordingly radiates energy inwardly to maintain the desired chemical reaction within the protective fluid-wall blanket.
Heat generating reactors have been employed in various industrial processes, but conventional reactors utilizing convection and/or conduction generally are not suitable for many high temperature reactions. At elevated temperatures, the heat transfer material for such conventional reactors generally reacts with the feedstock and fails, or reactants accumulate on the heat transfer walls of such conventional reactors, thereby substantially reducing the efficiency of the thermal process. Examples of such conventional reactors are found in U.S. Pat. Nos. 2,769,772, 2,926,073, and 3,565,766.
U.S. Pat. No. 2,750,260 discloses a technique for manufacturing titanium dioxide pigment by combustion of titanium tetrachloride with oxygen. The deposition of titanium dioxide particles on the walls of the cooling zone of the reaction apparatus is suppressed by slowly diffusing a purge gas through a porous wall. The mass of the purged gas is considered controlling rather than the gas pressure or pore size of the interstices through the wall, although the pore size less than 1 mm in diameter is preferred. In the combustion zone temperatures may reach 2940.degree. F. (1600.degree. C.), although in the cooling zone where the reactant deposition is suppressed, the temperature is in the range of only 1140.degree. F. (600.degree. C).
U.S. Pat. No. 3,499,730 discloses a combustion reactor for producing pigmentary titanium dioxide by the combustion of titanium tetrachloride in the presence of an auxiliary flame. The combustion reaction is carried out in a central flame confined within a foraminous wall tube. In the reaction zone, the cooling zone, and the recycling zone, a selected gas passes through the foraminous wall of the tube to form a barrier layer of gas which keeps particles of titanium dixoide formed in the central flame away from the wall of the tube. The gas passing through the foraminous wall of the tube also prevents the tube from reaching the temperature of the reaction zone, so that the wall may be maintained at substantially room temperature. The foraminous wall of the tube may thus be made of steel, aluminum or other metal, and perforations in the tube consists of from 2% to 40% of the wall surface.
U.S. Pat. No. 4,044,117 to Matovich discloses a fluid-wall reactor for carrying out chemical reactions at temperatures significantly higher than that disclosed in the earlier referenced patents. The reactor includes electrodes surrounded by a heat shield for heating a reactor tube made of a refractory material. The reactor tube is heated to a high temperature, and in turn emits sufficient radiant energy to initiate and sustain a desired chemical reaction which occurs within the interior of the reaction tube. Since the Matovich technique employs radiant energy rather than conduction or convection, a reactor tube material having poor thermal conductivity but relatively high temperature integrity may be utilized to form the reaction tube. This "radiation coupling" technique and its advantages over conventional reactors are fully discussed in the '117 patent.
In an attempt to alleviate the reactant and tube material reaction, and to reduce the accummulation of reactants on the interior surface of the reaction tube, Matovich utilizes a porous tube material to permit an inert gas to pass through the tube and provide a protective fluid wall for the inner surface of the tube. Various wall construction materials and types of perforations are disclosed in the '117 patent, although the reactor tube is preferably made of a porous refractory material having pore diameters in the range of from about 0.001 to 0.020 in. (0.025 mm to 0.508 mm).
The reactor disclosed in the '117 patent is thus considered a significant advancement over the prior art, since it teaches the generation of substantially increased reaction temperatures without significant deposition of reactants on the inner surface of the reactor tube. A suitable porous tube for such a reactor may be fabricated from graphite, with a wall thickness of about 0.75 in. (19 mm). Porous graphite is usually made by sintering particles of graphite coated with a carbonizable adhesive to bond the particles together. Interstices between the particles form a network of random pores though which gas can pass. It is both difficult and expensive, however, to maintain uniform porosity of the graphite material. In order to maintain reasonable porosity uniformity, the porous graphite tubes may be produced in small sections or blocks having a thickness greater than that desired. Known commercially available porous graphite tubes, not specifically intended for fluid wall reactors, are limited to a diameter of approximately 14 in. (356 cm). Since the desired fluid wall within the tube may be approximately 1 to 2 in. thick (25 to 51 mm) thick, the diameter of the inner reaction zone is limited to sizes which are frequently not commercially attractive.
The reactor of the '117 patent, although generally considered adequate for small-scale experimental work, has a number of significant drawbacks when operated on an industrial scale. Perhaps the most serious drawback is its inability to maintain a sufficient fluid wall to prevent reactants from contacting and reacting with the inner surface of the tube, especially at the location where the feedstock is input to the reaction zone. At commercial feedstock input rates, the reactants and porous tube chemically react, thus substantially reducing the life of the reaction tube. Moreover, engagement of the reactants with the porous tube tends to plug the interstices through the porous tube, thereby further reducing the effectiveness of the fluid wall and decreasing tube life.
In industrial applications, the porous reactor tube of Matovich also is not capable of efficiently transmitting heat to the reaction zone in order to sustain many desired chemical reactions. For example, when a hydrocarbon oil was introduced into a commercial embodiment of the reactor at a relatively low flow rate, oil may be decomposed into hydrogen and a high-quality carbon black. However, when it was attempted to increase the flow to a reasonable production rate, the produced carbon black became oily and otherwise deteriorated in quality. It was found that although the electrodes of the reactor were operating at a temperature of about 4000.degree. F. (2200.degree. C.), the temperature of the inner surface of the reactor tube was only about 3100.degree. F. (1700.degree. C.). Since the intensity of radiant energy emitted by a surface is approximately porportional to the absolute temperature raised to the fourth power, this decrease causes a reduction in the intensity of radiant energy by a factor of roughly 0.4. As a result of the reduced intensity of radiant energy in the reaction chamber, the decomposition reaction did not proceed to completion, thereby giving rise to carbon black of inferior quality. Also, after a relatively short reaction time of several hours, the porous tube failed, largely due to chemical reactions between the porous wall material and both the reactants and injected gas for the fluid wall. Some of the pores through the tube were plugged, and thus it was determined that the porous tube did not effectively maintain the desired fluid wall.
The number of chemical reactions which can be sufficiently commercialized with a reactor utilizing the porous tube of Matovich is therefore limited, because of its inability to maintain an effective fluid wall (which severely limits the life of the reactor tube), and because of its thermal inefficiency (due to the difference between the electrode temperature and the reaction temperature).
Further limitations to the Matovich reactor include the significant consumption of inert gas required to maintain a reasonable fluid wall. Significant consumption of nitrogen, a suitable inert gas, increases the reactor operating costs and demands large capacity downstream equipment for purifying the nitrogen for discharge. At elevated temperatures, nitrogen is not totally inert and does react with the graphite tube material, which structurally weakens the tube. Because of its porous nature, this nitrogen/graphite reaction occurs throughout the tube, and substantially limits the effective life of a porous tube. Finally, the nitrogen passing through the porous tube reaches substantial thermal equilibrium with the tube, thereby increasing the exit gas temperature which, in some reaction processes, must then be subsequently lowered.
The disadvantages of the prior art are overcome by the present invention, and an improved high temperature fluid wall reactor is hereinafter disclosed utilizing a perforated reactor tube. Also disclosed is an improved process for forming an effective fluid wall within a high-temperature reactor by forming a plurality of gas jets which overlap to form the protective fluid wall but do not disturb flow within the reaction zone.